Fast neutrinos, C-P violations, and the shrinking space for the Higgs

It's been a busy week in physics, as there have been updates on the hunt for …

It has been a busy week in the world of particle physics, with attention focused on the home of the LHC: CERN. This year, the LHC generated five inverse femtobarns worth of data—nearly half the amount generated during the entire lifetime of the Tevatron—before shutting down the proton program a few weeks ago. From now until its scheduled winter shutdown, the LHC will be doing lead ion collisions to examine the quark-gluon interactions that dominated the Universe immediately after the Big Bang.

In the mean time, analysis of the data has continued, and some significant news has come out this week. A further dissection of last year's data has placed tighter limits on where the Higgs boson, which provides mass to other particles, might be hiding (assuming it exists). Meanwhile, the LHCb detector, which studies particles that contain heavy quarks, has found an anomalous behavior that might hint at physics beyond the Standard Model. And the LHC accelerator chain has sent some more neutrinos to detectors at Italy's Gran Sasso, which has helped them eliminate some potential sources of error in their faster-than-light findings. We'll take a look at each of these in turn.

Analysis of the LHC's proton collisions takes time and a very large computing grid. The results of last year's run were only analyzed in the summer, with both the ATLAS and CMS detectors providing an independent analysis of about 2.3 inverse femtobarns (less than half of what they now have to work with). Now, the teams have combined their data in order to get a better handle on the Higgs. Their combined results leave the area below 141GeV open as a possible hiding place for the particle, but excludes a huge region at higher energies.

Statistically, combining the two sets of results isn't a simple thing, since the detectors are physically different, and have slightly different properties. Now that a procedure has been worked out for handling it, that will hopefully smooth the process of performing a similar analysis with this year's data. However, as Symmetry Breakingnotes, with over 7 inverse femtobarns in total, each detector may have the chance to make a splash on its own, and we'll probably see a bit of healthy competition, at least initially.

And if that doesn't pan out, there are apparently versions of the Higgs that don't fit within the Standard Model. Meaning that, if this search comes up dry, the physics community won't run out of things to look for.

Meanwhile, another detector at the LHC may have found another hint that the Standard Model may come up a bit short. The LHCb detector specializes in the detection of particles called mesons, which contain a heavy quark (the "b" in its name refers to "beauty," another name for the bottom quark). In this case, it was examining the decay of particles that contain charm quarks and antiquarks, called D mesons.

Based on the Standard Model, the decay of the particles and their antiparticles should occur at similar rates. But LHCb has measured a difference between them that is several standard deviations away from that prediction. Indications that matter and antimatter behave differently are called C-P violations; the Standard Model can accomodate some level of C-P violation, but only so much. Right now, it's not clear whether this new finding is enough to break the Standard Model or explain the excess of matter in our Universe, but it's definitely something they'll be trying to refine the measurement on.

Not everything that CERN does ends up at the LHC. Part of its accelerator chain can be directed to producing neutrinos, which are then detected at Gran Sasso in Italy. These are the ones that are famously arriving a touch more quickly than light does. A key part of that analysis was assigning the neutrinos that showed up to a specific portion of a proton bunch that was produced by the SPS accelerator at CERN.

These bunches take a few nanoseconds to completely smash into the target that makes neutrinos, providing a potential source of error; if a neutrino was assigned to the wrong bunch, or wrong portion of a bunch, then the faster-than-light measurement would be completely off. To handle this issue, the CERN staff has worked on making very short, widely spaced bunches of protons in the SPS. The full length of these bunches is three nanoseconds, much shorter than the 60 nanosecond lead the neutrinos had on light. And the bunches have been separated by over 500 nanoseconds, eliminating the potential for misassignment.

With these changes in place, Gran Sasso has now collected 20 more events and shown that their original analysis still holds under these conditions. They've updated their paper on the arXiv accordingly.